1. Field of the Invention
The present invention relates to an information recording medium on which information is recorded by using a wobble, and an apparatus for and a method of reproducing information.
2. Description of the Related Art
In an optical disc represented by a Compact Disc (CD) and a DVD, the long or short length of a pit is used to record information. However, in order to record information for copy control for the prevention of illegal copying or the like, there is a request for reserving another recording area without decreasing a recording capacity by the pit.
As a method of increasing the recording capacity by other means except the method using the long or short length of the pit, there is known a technique of displacing or shifting the position of the pit in the radial direction of the optical disc. This technique is that information is recorded by wobbling the position of the pit in the radial direction of the optical disc and by performing spread spectrum with respect to the wobble (e.g. Japanese Patent Application Laying Open NO. 2003-85896).
This technique is such that a wobble signal is generated by performing spread spectrum with respect to predetermined data with random data and that the position of a record mark is wobbled according to the wobble signal. Since the frequency component of the wobble is expanded by spread spectrum, it is possible to decrease crosstalk from an adjacent track to some extent.
However, if spread spectrum is performed by using the random data, “0” or “1” long continues or appears disproportionately with some probability. In this case, the record mark is formed at a position out of the center of the track. If such a record mark that is out of the center of the track is read, an optical pickup moves to follow the deviation of the record mark, thereby to generate a tracking offset. As a result, problems occur, such as tracking error by jumping to the adjacent track and the decrease of data reliability by crosstalk from the adjacent track. These are big problems, particularly in an optical disc with a high recording density and a narrow track pitch.